X-ray computed tomography apparatuses are frequently used for evaluation of the coronary lumen of the heart. A method of removing a calcification region and a stent region by subtracting a non-contrast CT image from a contrast CT image to reduce the influence of the calcification region and stent on the evaluation of lumen is known. In subtraction processing for a heart region, motions of the calcification region and the stent region accompanying pulsation of the heart exert an enormous influence on the image quality of subtraction images. Further, the combination of volume images intended for subtraction processing has an enormous influence on the image quality of subtraction images. If, for example, geometrical features such as the position, shape, or CT value of the calcification region and stent region are different between a non-contrast CT image and a contrast CT image, the image quality of a subtraction image is degraded. An observer judges the combination of a non-contrast CT image and a contrast CT image by visual inspection. Thus, the work flow related to subtraction processing is prolonged. In addition, empirical knowledge is needed for the selection of the optimum combination by visual inspection and thus, the combination and subtraction images vary among individuals.